To a medium skillet, on medium heat, add onion and garlic and cook, stirring, until it is soft and translucent, about 5 minutes. Add the ground beef, breaking it up with the back of a spoon, and cook until browned, about 10 minutes. Reduce heat to low, and stir in ketchup, chili powder, and mustard. Cover with lid simmer for 15 minutes until thickened. Season with salt and pepper, to taste.